DE102016207303A1 - Solar cell and method of manufacture - Google Patents

Abstract

The present invention relates to a solar cell having a silicon substrate having a substrate backside, wherein on the back of the substrate, a gallium back field is provided. The present invention further relates to various variants of a method for producing such a solar cell.

Description

FIELD OF THE INVENTION

The present invention relates to a solar cell and a method for producing a solar cell.

TECHNICAL BACKGROUND

Solar cells usually have a silicon substrate which has a doped base. For example, a region with p-doping atoms, for example boron (B), gallium (Ga) or aluminum (Al) may be provided. Alternatively, the base may be doped with n-doping atoms, for example phosphorus (P) or arsenic (As).

PERC (Passivated Emitter and Rear Cell) refers to a solar cell technology with which significantly higher efficiencies can be achieved. In a PERC solar cell, the silicon substrate has a structured passivation layer on the back side of the semiconductor body, which is intended to reduce recombination losses at the backside contact of the solar cell. The associated contact structure is arranged on the passivation layer and contacts the back surface of the semiconductor body in a local manner via contact openings present in the passivation layer. Such a PERC solar cell is z. B. in the EN 20 2015 101 360 U1 described.

Aluminum BSF (BSF: Back Surface Field), also known as aluminum back field, denotes a solar cell design, wherein an aluminum layer is provided directly on the back of the silicon substrate. In the area of the layer boundary with silicon, an aluminum-silicon alloy is formed in the production during a fire process. Since aluminum can act as a dopant in silicon, by means of the recrystallized silicon with built-in aluminum, a so-called Al-BSF (aluminum back-surface field or aluminum back surface field) is formed. The task of the aluminum BSF is to prevent generated electrons from recombining at the back. This is achieved by the p + doping of the trivalent aluminum.

Furthermore, there are PERC solar cells in which an aluminum BSF is formed locally in the region of recesses of the passivation layer, in which a metallization layer made of aluminum contacts the substrate. For example, this describes DE 20 2015 103 803 U1 a PERC solar cell with local aluminum BSF.

However, aluminum has (also) unfavorable recombination properties. In particular, aluminum can form complexes with oxygen, which can lead to the recombination of charge carriers.

In addition, to reduce the effect of light-induced degradation (LID), sometimes gallium doping of the substrate or crystal is often required to bind copper atoms responsible for the LID.

This is a condition to be improved.

SUMMARY OF THE INVENTION

Against this background, the present invention has the object to provide an improved solar cell.

According to the invention, this object is achieved by a solar cell having the features of patent claim 1 and / or by a method having the features of patent claim 8 and / or by a method having the features of patent claim 9 and / or by a method having the features of patent claim 12 ,

• – Eine Solarzelle, mit einem Silizium-Substrat, welches eine Substratrückseite aufweist, wobei an der Substratrückseite ein Gallium-Rückseitenfeld vorgesehen ist.
• – Ein Verfahren zur Herstellung einer Solarzelle, insbesondere einer erfindungsgemäßen Solarzelle, mit den Schritten: Aufbringen von Gallium auf eine mit Ausnehmungen strukturierte Passivierschicht, welche an der Rückseite eines Silizium-Substrats vorgesehen ist; und Eintreiben des Galliums als Dotierung in das Silizium-Substrat in dem Bereich der Ausnehmungen zur Bildung eines lokalen Gallium-Rückseitenfeldes.
• – Ein Verfahren zur Herstellung einer Solarzelle, insbesondere einer erfindungsgemäßen Solarzelle, mit den Schritten: Aufbringen von Gallium vollflächig direkt auf die Substratrückseite eines Silizium-Substrats; und Eintreiben des Galliums als Dotierung in das Silizium-Substrat zur Bildung eines Gallium-Rückseitenfeldes.
• – Ein Verfahren zur Herstellung einer Solarzelle, insbesondere einer erfindungsgemäßen Solarzelle, mit den Schritten: Aufbringen einer eine Galliumverbindung aufweisenden Passivierschicht direkt auf die Substratrückseite eines Siliziumsubstrats; Eintreiben von Gallium aus der Passivierschicht als Dotierung in das Silizium-Substrat zur Bildung eines Gallium-Rückseitenfeldes; und Entfernen von Rückständen der Galliumverbindung.

Accordingly, it is provided:

• - A solar cell, with a silicon substrate, which has a substrate back, wherein on the back of the substrate, a gallium back field is provided.
• A method for producing a solar cell, in particular a solar cell according to the invention, comprising the steps of: depositing gallium on a recess-structured passivation layer which is provided on the back side of a silicon substrate; and driving the gallium as doping into the silicon substrate in the region of the recesses to form a local gallium back surface field.
• A method for producing a solar cell, in particular a solar cell according to the invention, comprising the steps of: depositing gallium over the entire surface directly onto the substrate rear side of a silicon substrate; and driving the gallium as a dopant into the silicon substrate to form a gallium back surface field.
• A method for producing a solar cell, in particular a solar cell according to the invention, comprising the steps of: depositing a gallium compound-comprising passivation layer directly on the substrate back side of a silicon substrate; Driving gallium out of the passivation layer as a dopant into the silicon substrate to form a gallium backside field; and removing residues of the gallium compound.

The idea underlying the present invention is to form a back surface field or a BSF with gallium. Gallium is less reactive than aluminum. In this way, the unfavorable recombination properties of aluminum, which is commonly used for a BSF, are avoided. Furthermore, gallium doping of the silicon substrate base or of the crystal for binding copper atoms can become obsolete.

The finding underlying the present invention is that gallium also forms a eutectic with silicon, can act as a dopant in silicon and also has high temperature-dependent effective diffusion constants. By means of gallium incorporated into the silicon substrate, it is therefore possible to form a gallium back surface field or gallium BSF with comparatively low production costs. This would be possible, for example, with the also trivalent element boron due to the very slow diffusion of boron into silicon and the absence of a eutectic in the boron-silicon system only with much greater effort.

Advantageously, according to the invention, electrons are effectively prevented from recombining at the backside of the substrate by the gallium backside field, which is achieved by the p + doping of the trivalent gallium. This is advantageously achieved without aluminum, which itself represents a recombination center. Thus, it is advantageous to avoid the partial compensation of the positive effect of the rear side field which is present in the case of aluminum.

The gallium back surface field is to be distinguished from a conventionally gallium doped silicon crystal. A significant difference lies above all in the achievable concentration of gallium. While a conventional gallium doped wafer or silicon crystal has concentrations of 10 ¹⁴ to 10 ¹⁷ atoms per cubic centimeter, much higher concentrations can be achieved with a gallium back surface field of the present invention.

Another difference is the distribution of gallium, which is much more heterogeneous with a gallium back surface field and concentrates on a topmost layer of the silicon substrate. However, this is harmless since copper diffuses very rapidly at high temperatures in silicon. Therefore, the entire volume of the silicon substrate need not be highly doped with gallium because the copper can also diffuse to the gallium. In this way, stable copper-gallium pairs are formed without the need for conventional gallium doping of the silicon crystal, thus reducing the LID effect.

Further, the gallium backface is to be distinguished from a passivation layer with a gallium compound and also from metallization with gallium-containing pastes. Although these may also have gallium driven into the silicon, the difference lies in the penetration depth of the gallium. In the case of a gallium back field, the penetration depth, in contrast to the passivation or metallization, is significantly more than 500 nanometers in the uppermost layer of a silicon substrate. For example, penetration depths greater than 2 micrometers are possible and theoretically up to 20 microns conceivable. This is inventively achievable by targeted driving of the gallium.

The driving of the gallium can be done in different ways.

On the one hand, it can either be limited only locally, that is to say on predetermined surface sections of the substrate rear side, or be driven over the entire surface. Furthermore, different starting materials, such as pure gallium, gallium alloys or gallium compounds, possible.

Only local application can be achieved by applying gallium to a passivation layer or passivation layer provided with recesses and then driving it in.

Alternatively, the gallium can be applied directly to the substrate, in particular over the entire surface, and driven in.

Gallium residues which have not been driven into the silicon substrate as doping and any gallium-silicon alloy formed during driving are preferably removed after driving. In particular, this can be done, for example, a Rückätzen the gallium residues. This is due to the fact that in particular gallium-silicon alloys form a eutectic and thus have a very low melting point. With residues of gallium, the substrate would thus be very difficult to process further to a solar cell. In particular, then a metallization, which usually higher temperatures are necessary, would hardly be possible. On the other hand, the gallium backside field is not affected by the higher temperatures, since here the gallium is present only as doping in the silicon.

As an alternative to removing the residues of gallium, it would also be conceivable to completely oxidize the gallium during or with the driving in and to use a gallium oxide layer formed as a passivation layer.

Another possibility is to apply the gallium in the form of a passivating layer having a gallium compound on the back of the substrate. The gallium can then be driven out of the passivation layer into the substrate. Subsequently, the residues of the gallium compound can be removed. For example, the gallium compound may be gallium (III) oxide (Ga ₂ O ₃ ) and / or gallium trichloride (GaCl ₃ ). The passivation layer is thus a "sacrificial passivation layer".

In principle, both monocrystalline and multicrystalline substrates are suitable as the silicon substrate.

Furthermore, it is conceivable to provide a backside passivation, for example with silicon nitride, to form a PERC solar cell in addition to the gallium back surface field.

The application of gallium can be done in different ways. For example, the gallium may be deposited by vapor deposition by CVD (Chemical Vapor Deposition) or PVD (Physical Vapor Deposition).

In addition, the gallium can also be applied by means of spin coating (coating). Advantageously, a wafer only needs to be heated to 30 ° C, since gallium melts at this temperature and thus distributed by the centrifugal forces during rotation.

Furthermore, it is also conceivable to apply the gallium by vapor deposition, sputtering or printing, for example by means of screen printing or inkjet printing, and application in paste form or by dipping.

Also conceivable is the application of gallium by contact with a gallium-containing atmosphere or with a gallium-containing glass. Another conceivable option would be ion implantation.

Metallization of a gallium backface arrayed substrate is preferably done without aluminum because an aluminum-silicon eutectic on the backside could damage or destroy the gallium backface. Thus, preferably other metals are used. For example, a metallization can be provided by means of silver-containing pastes.

Advantageous embodiments and further developments will become apparent from the other dependent claims and from the description with reference to the figures of the drawing.

In one embodiment of a solar cell, the gallium back surface field has gallium doping to a depth greater than 500 nanometers in the silicon substrate. Preferably, the gallium doping is provided to a depth greater than 2 microns (μm). For example, the gallium doping can be provided to a depth of 2 to 5 microns. Also conceivable are depths of gallium doping greater than 5 microns, especially up to 20 microns. In this way, high total concentrations of gallium in the substrate are advantageously made possible by forming the gallium back surface field.

According to one embodiment, a passivation layer is provided on the substrate backside. In particular, the solar cell can therefore be a PERC solar cell. The efficiency of the solar cell is thus advantageously increased. If the passivation layer is applied before the gallium is driven in, it can be provided with a structuring or with recesses, in particular by means of laser ablation, and the gallium can be locally driven in the area of the recesses. If the passivation is applied after driving in the gallium, the gallium can be driven over the entire surface. In one embodiment - in this case, after the driving in, all the gallium residues are removed before the application of the passivation layer, for example by etching back.

According to a further embodiment, the passivation layer is formed as a gallium oxide layer. In this way, the gallium back surface field can be made by driving gallium into the silicon substrate without having to subsequently remove the gallium oxide layer. In particular, the gallium may also be oxidized upon driving so that the gallium oxide layer is formed in one step with driving. Thus, advantageously, the number of process steps for the production of the solar cell can be reduced.

According to a preferred embodiment, the silicon substrate has a gallium concentration greater than 10 ¹⁷ atoms per cubic centimeter. Preferably, the concentration is greater than 10 ¹⁸ atoms per cubic centimeter. Particularly high concentrations are possible when an already conventionally gallium-doped wafer is used as the silicon substrate, and in addition the gallium back surface field is provided.

In one embodiment, the gallium backplate is provided all over the topmost layer on the substrate backside of the silicon substrate. In this way, advantageously high total concentrations of gallium in the silicon substrate are possible.

According to a further embodiment, the gallium back surface field is only partially provided locally in the uppermost layer on the substrate rear side of the silicon substrate. In particular, the local portions of the gallium backside field are the area of recesses of a passivation layer. The recesses are introduced into the passivation layer, for example by means of laser ablation. Advantageously, in this way a PERC solar cell can be provided with high efficiency and without the negative recombination properties of aluminum.

According to one embodiment of a method, after depletion, a step of removing residues of gallium is provided. Alternatively or additionally, in the step of removing residues, a gallium-silicon alloy is removed which has or may be generated during the driving. Advantageously, the substrate can thus be further processed in a conventional manner, without having to take into account the low melting point of gallium or gallium-silicon eutectic consideration.

In one embodiment, the gallium is oxidized during driving to form a passivation layer. In particular, a full-surface Ga ₂ O ₃ layer is formed. Thus, it is particularly advantageous to simultaneously drive the gallium into the substrate to form a gallium back surface field and to form a passivation layer. Advantageously, therefore, a further step of applying a passivation layer can be dispensed with.

In one embodiment, driving the gallium into the silicon substrate includes high temperature annealing, laser beam, flash lamp, and / or firing. For example, pure gallium can be driven by annealing, firing, flash lamp or laser. For example, from a passivation layer containing a gallium compound, the gallium may be thermally, i. H. be driven by high temperature, by laser, or by flash lamp in the silicon substrate. If the gallium is to be oxidized on driving to a passivation layer, the driving is preferably carried out thermally.

In one embodiment, the gallium is driven in driving to a depth greater than 500 nanometers. In particular, the depth is greater than 2 microns.

The above embodiments and developments can, if appropriate, combine with each other as desired. In particular, all the features of a solar cell can be transferred to a process for producing the solar cell, and vice versa.

Further possible refinements, developments and implementations of the invention also include combinations, not explicitly mentioned, of features of the invention described above or below with regard to the exemplary embodiments. In particular, the person skilled in the art will also add individual aspects as improvements or additions to the respective basic form of the present invention.

CONTENT OF THE DRAWING

The present invention will be explained in more detail with reference to the exemplary embodiments indicated in the schematic figures of the drawing. It shows:

1 A phase diagram of the system silicon gallium;

2A a silicon substrate with gallium layer applied;

2 B the silicon substrate according to 2A after driving in the gallium;

3A - 3H Steps for manufacturing a solar cell according to a first embodiment;

4A - 4F Steps for producing a solar cell according to a second embodiment;

5A - 5H Steps for producing a solar cell according to a third embodiment;

6A - 6F Steps for manufacturing a solar cell according to a fourth embodiment;

The accompanying figures of the drawing are intended to convey a further understanding of the embodiments of the invention. They illustrate embodiments and, together with the description, serve to explain principles and concepts of the invention. Other embodiments and many of the stated advantages will become apparent with reference to the drawings. The elements of the drawings are not necessarily shown to scale to each other.

In the figures of the drawing are the same, functionally identical and same-acting elements, features and components - unless otherwise stated - each provided with the same reference numerals.

DESCRIPTION OF EMBODIMENTS

1 shows a phase diagram of the system silicon gallium.

On the right axis or abscissa axis, the concentration ratio between gallium and silicon is plotted, the lower scale represents the atomic percentage and the upper scale represents the weight percentage of gallium.

On the vertical axis or ordinate axis the temperature is plotted in degrees Celsius.

The solidus line S of the system runs at concentrations up to 90 atomic percent gallium above 1000 ° C. Nevertheless, with a share of 99.994 atomic percent gallium, there is a eutectic E, ie a low-melting liquid phase, at a temperature T _SE of just below 30 ° C. (29.77 Celsius).

Because of this property of the silicon-gallium system, mixing of the elements at relatively low temperatures is possible, albeit with very little but sufficient solubility of gallium in silicon.

Furthermore, gallium has a comparatively high diffusion constant in silicon at lower temperatures. The following table shows the different effective diffusion constants D _{eff of} different dopants in silicon at a certain temperature taking into account the activation energy. As a comparison, the diffusion constants of phosphorus (P), boron (B) and gallium (Ga) in silicon are compared: D _eff [cm ² / s] D _{820 ° c} D _{850 ° c} D _{900 ° c} D _{1000 ° C} P 2,31E-16 6,06E-16 2,71E-15 3,80E-14 B 3,52E-18 1.05E-17 5,80E-17 1,17E-15 ga 6,94E-17 2,09E-16 1,16E-15 2,38E-14

Accordingly, phosphorus at a temperature of 820 ° C, which corresponds to the temperature of an emitter diffusion, has a value which is also reached by gallium at 850 ° C. Boron, on the other hand, requires significantly higher temperatures for such a diffusion constant.

The comparison shows that, from a thermodynamic point of view, diffusion of gallium is much preferable to that of boron.

2A shows a silicon substrate 2 with applied gallium layer.

It is a for the production of a solar cell 1 provided silicon substrate 2 with a thickness d. For example, the thickness d is 160 μm.

At a substrate backside 3 of the silicon substrate 2 is a layer of gallium 8th applied over the entire surface. For example, this layer may be gallium 8th have a thickness of 1 micron.

2 B shows the silicon substrate 2 according to 2A after driving in the gallium 8th ,

The driving in is here with the wavy arrows between 2A and 2 B symbolizes and is made at a temperature T of for example 1200 ° C.

By driving it has become in the silicon substrate 2 at its substrate back 3 a gallium back field 4 that is, a section heavily doped with gallium.

In this way, a solar cell can be made, which at the back of the substrate 3 of the silicon substrate 2 a gallium back field 4 having.

The 3A - 3H show steps to make a solar cell 1 according to a first embodiment.

In this embodiment, it is a PERC process, that is, a manufacturing process of a solar cell with local formation of a gallium back surface field (GA-BSF) in a single crystal wafer. For example, you may be dealing with a CZ wafer, FZ wafer, quasi-monocrystalline wafer, or the like.

Of course, a transfer of the embodiment to multicrystalline (mc) wafers is also possible.

3A shows a step of providing a silicon substrate 2 ,

In particular, this includes a step of cleaning or sawing damage sets of a wafer. This is provided with an alkaline texture and subjected to emitter diffusion. In addition, the steps of the edge insulation, the Phosphorglasätze and the back side polishing take place. In one embodiment, these steps may be performed the same as in a conventional manufacturing process of a substrate for a solar cell.

3B shows a subsequent step of backside passivation. This is a passivation layer 6 directly onto the surface of the back of the substrate 3 applied. The passivation layer is, for example, silicon nitride (SiN or Si ₃ N ₄ ).

3C shows a step of structuring the passivation layer 6 , This is preferably carried out by laser ablation, wherein a recess 9 into the passivation layer 6 is introduced.

The two steps according to 3B and 3C can be made in one embodiment the same as for the production of a conventional PERC solar cell.

3D shows a step of the full-surface application of gallium. This step represents a difference of the process to the manufacturing process of conventional PERC solar cells.

The full-surface application of gallium can be done in different ways, for example by PVD, CVD, screen printing or inkjet printing. The recess 9 is doing with gallium 8th filled.

3E shows the step of driving in the gallium.

This step also represents a difference of the method to the production process of conventional PERC solar cells.

The driving takes place here, for example, thermally, d. H. by annealing the substrate. Alternatively or additionally to the annealing, a laser treatment for driving in can also be provided.

When driving a eutectic forms, d. H. a liquid phase of a gallium-silicon compound, and diffusion of gallium atoms into the silicon substrate takes place. The diffusion process is accelerated and intensified by the liquefaction.

As the melt solidifies, a segregation process takes place, with the gallium backside field 4 and form gallium-silicon mixed phases or alloys.

In this way, the pure silicon substrate 2 to a depth 5 of> 500 nm, for example 2 μm, doped with gallium atoms. Thus, a gallium back field becomes 4 educated.

3F shows a step of removing residues 8th' of gallium and gallium-silicon alloy formed during driving.

This step also represents a difference of the method to the production process of conventional PERC solar cells. For example, the removal of the residues takes place 8th' by re-etching.

After removing the residues 8th' now only the gallium back field remains 4 that is, silicon substrate doped with gallium atoms 2 , Incidentally, all gallium is removed.

3G shows a step of metallizing the substrate backside 3 ,

It is here only the area of the back of the substrate 3 shown, wherein a portion of a front side is not shown.

As in the manufacturing process of a conventional PERC solar cell, the front side, not shown, is treated before or after the illustrated step of metallizing the back of the substrate. For this purpose, the front side, not shown, is usually provided with a selective emitter and applied an antireflection layer, for example of silicon nitride, on the front. Furthermore, a metallization of the front side by means of applying a so-called grid by screen printing, applying silver pads for contacting and firing of the substrate is made.

The here in 3 illustrated metallization of the back is preferably carried out by a so-called "plating" process. For this purpose, a starting layer is first applied to the surface to be metallized. This can, for. B. from electroless nickel deposited ("electroless nickel") or from colloidal palladium. For this purpose, the gallium-doped wafer is immersed in a corresponding bath which contains a nickel salt (eg NiCl ₂ ) and a reducing agent (eg NaH ₂ PO ₂ ). When the bath is heated, the sodium hypophosphite reduces the Ni ²⁺ ion to metallic nickel, which on the wafer surface acts as a nickel layer 13 separates.

In the case of the presence of a passivation layer 6 As in the present embodiment, the deposition takes place only in the region of the recesses 9 ,

For achieving the transverse conductivity is on the nickel layer 13 Deposited copper or silver. In the embodiment shown, in this way a copper layer 14 on the nickel layer 13 applied.

For better solderability and to prevent corrosion, the copper layer 14 again by means of a galvanic or electroless deposited topcoat 15 For example, from silver (Ag), tin (Sn) or nickel (Ni), completed.

For contact formation, the wafer is heated to about 200-400 ° C, forming a very good contacting nickel silicide. The thermal treatment can be carried out immediately after the nickel plating, but advantageously also after the deposition of the total layer 13 . 14 . 15 respectively.

Thus, the solar cell 1 completed.

As an alternative to a plating process, the backside metallization can also be done by vapor deposition or screen printing of non-firing paste.

• • Sägeschadenätze
• • Alkalische Textur
• • Emitterdiffusion
• • Kantenisolation/Phosphorglasätze/Rückseitenpolitur
• • Rückseitenpassivierung (SiN)
• • Laserablation
• • Aufbringen von vollflächigem Ga (PVD/CVD/Siebdruck/Tintenstrahldruck)
• • Eintreiben des Ga aus der Oxidschicht (Thermisch, Laser) zur Bildung eines BSFs mit Dicke > 500 nm über ein Eutektikum
• • Rückätzen von Ga-Rückständen
• • Selektiver Emitter
• • Antireflexschicht auf der Vorderseite (SiN)
• • Siebdruck Vorderseiten-Grid/Feuern
• • Platen von Nickel/Bildung von Ni_xSi_y thermisch/Platen von Kupfer
• • Deckschicht Zinn zum Löten

Thus, according to this embodiment, the manufacturing method may include, by way of example, the following steps: Example of a PERC process (single crystal wafer) with local Ga-BSF (gallium back field) formation

• • Saw damage sets
• • Alkaline texture
• • emitter diffusion
• • Edge insulation / Phosphor glass etchings / Backside polish
• Backside passivation (SiN)
• • laser ablation
• Apply Full Ga (PVD / CVD / Screen Printing / Inkjet Printing)
• • Driving the Ga out of the oxide layer (thermal, laser) to form a BSF with thickness> 500 nm via a eutectic
• • Back etching of Ga residues
• • Selective emitter
• • Anti-reflection layer on the front (SiN)
• • Screen printing front side grid / firing
• Plating of nickel / formation of Ni _x Si _y thermal / plating of copper
• • Cover tin for soldering

• • Saure Textur
• • Emitterdiffusion
• • Kantenisolation/Phosphorglasätze/Rückseitenpolitur
• • Rückseitenpassivierung SiN
• • Laserablation
• • Aufbringen von vollflächigem Ga (PVD/CVD/Siebdruck/Tintenstrahldruck)
• • Eintreiben des Ga aus der Oxidschicht (Thermisch, Laser) zur Bildung eines BSFs mit Dicke > 500 nm über ein Eutektikum
• • Rückätzen von Ga-Rückständen
• • Selektiver Emitter
• • Antireflexschicht auf der Vorderseite (SiN)
• • Siebdruck Vorderseiten-Grid/Feuern
• • Platen von Nickel/Bildung von Ni_xSi_y thermisch/Platen von Kupfer
• • Deckschicht Zinn zum Löten

Example of a mc-PERC process (multicrystalline wafer) with local Ga-BSF formation

• • Acid texture
• • emitter diffusion
• • Edge insulation / Phosphor glass etchings / Backside polish
• • Backside passivation SiN
• • laser ablation
• Apply Full Ga (PVD / CVD / Screen Printing / Inkjet Printing)
• • Driving the Ga out of the oxide layer (thermal, laser) to form a BSF with thickness> 500 nm via a eutectic
• • Back etching of Ga residues
• • Selective emitter
• • Anti-reflection layer on the front (SiN)
• • Screen printing front side grid / firing
• Plating of nickel / formation of Ni _x Si _y thermal / plating of copper
• • Cover tin for soldering

The 4A - 4F show steps to make a solar cell 1' according to a second embodiment.

In contrast to the first embodiment, this is a solar cell 1' without backside passivation.

At the silicon substrate 2 This may also be a single crystal wafer or a multicrystalline wafer.

Here is the gallium 8th according to 4B full surface directly on the silicon substrate 2 applied. Accordingly, a gallium back field becomes 4 also over the entire surface over the entire back 3 generated, see 4 C. The driving can be done for example by firing and / or by means of a flashlamp.

Here, too, are the steps of removing debris 8th' of gallium and gallium-silicon alloy, see 4D , as well as metallizing the substrate back, as in the 4E and 4F shown, made.

• • Sägeschadenätze
• • Alkalische Textur
• • Emitterdiffusion
• • Kantenisolation/Phosphorglasätze/Rückseitenpolitur
• • Vorderseiten SiN (Antireflexschicht)
• • Aufbringen des Galliums vollflächig auf der Rückseite
• (PVD, CVD, Sputtern, Tauchen, Tintenstrahldruck, Siebdruck)
• • Eintreiben durch Feuern, Blitzlampe zur Bildung eines BSFs mit Dicke > 500 nm
• • Entfernen der Legierung aus Ga und Si
• • Siebdruck Vorderseiten-Grid/Feuern
• • Platen von Nickel/Bildung von Ni_xSi_y thermisch/Platen von Kupfer
• • Deckschicht Zinn zum Löten

Thus, according to this embodiment, the manufacturing method may comprise the following steps by way of example:
An Example of a CZ-Ga-BSF Process (Monocrystalline CZ Wafer with Gallium Back Side Panel)

• • Saw damage sets
• • Alkaline texture
• • emitter diffusion
• • Edge insulation / Phosphor glass etchings / Backside polish
• Front side SiN (antireflection layer)
• • Apply the gallium all over the back
• (PVD, CVD, sputtering, dipping, inkjet printing, screen printing)
• • Driving in by firing, flash lamp to form a BSF with thickness> 500 nm
• • Remove the alloy of Ga and Si
• • Screen printing front side grid / firing
• Plating of nickel / formation of Ni _x Si _y thermal / plating of copper
• • Cover tin for soldering

• • Saure Textur
• • Emitterdiffusion
• • Kantenisolation/Phosphorglasätze/Rückseitenpolitur
• • Vorderseiten SiN (Antireflexschicht)
• • Aufbringen des Galliums vollflächig auf der Rückseite (PVD, CVD, Sputtern, Tauchen, Tintenstrahldruck, Siebdruck)
• • Eintreiben durch Feuern, Blitzlampe zur Bildung eines BSFs mit Dicke > 500 nm
• • Entfernen der Legierung aus Ga und Si
• • Siebdruck Vorderseiten-Grid/Feuern
• • Platen von Nickel/Bildung von Ni_xSi_y thermisch/Platen von Kupfer
• • Deckschicht Zinn zum Löten

An example of an mc-Ga-BSF process (multi-crystalline gallium back surface field wafer)

• • Acid texture
• • emitter diffusion
• • Edge insulation / Phosphor glass etchings / Backside polish
• Front side SiN (antireflection layer)
• Full application of gallium on the reverse side (PVD, CVD, sputtering, dipping, inkjet printing, screen printing)
• • Driving in by firing, flash lamp to form a BSF with thickness> 500 nm
• • Remove the alloy of Ga and Si
• • Screen printing front side grid / firing
• Plating of nickel / formation of Ni _x Si _y thermal / plating of copper
• • Cover tin for soldering

The 5A - 5H show steps to make a solar cell 1'' according to a third embodiment.

This embodiment differs from the first two embodiments in that instead of a layer with gallium 8th a passivation layer 12 which is a gallium compound 11 contains, as Opferpassivierung on the silicon substrate 2 is applied, see 5B ,

For the gallium compound 11 it is, for example, gallium (III) oxide (Ga ₂ O ₃ ) and / or gallium trichloride (GaCl ₃ ).

At the silicon substrate 2 Again this may be a single crystal wafer or a multicrystalline wafer.

From the passivation layer 12 fires and / or flashes gallium into the silicon substrate 2 driven in, as in 5C shown. Subsequently, arrears 11 ' the gallium compound 11 removed, for example by etching back.

On the resulting full-surface gallium back field 4 then becomes a passivation layer in a conventional manner 6 upset, see 5E , which then with recesses 9 is structured, see 5F ,

Subsequently, a backside metallization is made, as in the 5G and 5H shown.

• • Sägeschadenätze
• • Alkalische Textur
• • Emitterdiffusion
• • Kantenisolation/Phosphorglasätze/Rückseitenpolitur
• • Opferpassivierung mit Ga2O3/GaCl3 (z. B. PVD/CVD)
• • Eintreiben durch Feuern, Blitzlampe zur Bildung eines BSFs mit Dicke > 500 nm
• • Rückätzen von Ga-Rückständen
• • Rückseitenpassivierung SiN
• • Laserablation
• • Selektiver Emitter
• • Antireflexschicht auf der Vorderseite (SiN)
• • Siebdruck Vorderseiten-Grid/Feuern
• • Platen von Nickel/Bildung von Ni_xSi_y thermisch/Platen von Kupfer
• • Deckschicht Zinn zum Löten

Thus, according to this embodiment, the manufacturing method may exemplarily include the following steps: In an example of a PERC process (single crystal wafer) with Ga 2 O 3 / GaCl 3 sacrificial passivation

• • Saw damage sets
• • Alkaline texture
• • emitter diffusion
• • Edge insulation / Phosphor glass etchings / Backside polish
• • sacrificial passivation with Ga2O3 / GaCl3 (eg PVD / CVD)
• • Driving in by firing, flash lamp to form a BSF with thickness> 500 nm
• • Back etching of Ga residues
• • Backside passivation SiN
• • laser ablation
• • Selective emitter
• • Anti-reflection layer on the front (SiN)
• • Screen printing front side grid / firing
• Plating of nickel / formation of Ni _x Si _y thermal / plating of copper
• • Cover tin for soldering

• • Saure Textur
• • Emitterdiffusion
• • Kantenisolation/Phosphorglasätze/Rückseitenpolitur
• • Opferpassivierung mit Ga2O3/GaCl3 (z. B. PVD/CVD)
• • Eintreiben durch Feuern, Blitzlampe zur Bildung eines BSFs mit Dicke > 500 nm
• • Rückätzen von Ga-Rückständen
• • Rückseitenpassivierung SiN
• • Laserablation
• • Selektiver Emitter
• • Antireflexschicht auf der Vorderseite (SiN)
• • Siebdruck Vorderseiten-Grid/Feuern
• • Platen von Nickel/Bildung von Ni_xSi_y thermisch/Platen von Kupfer
• • Deckschicht Zinn zum Löten

An example of a mc-PERC process (multicrystalline wafer) with Ga2O3 / GaCl3 sacrificial passivation

• • Acid texture
• • emitter diffusion
• • Edge insulation / Phosphor glass etchings / Backside polish
• • sacrificial passivation with Ga2O3 / GaCl3 (eg PVD / CVD)
• • Driving in by firing, flash lamp to form a BSF with thickness> 500 nm
• • Back etching of Ga residues
• • Backside passivation SiN
• • laser ablation
• • Selective emitter
• • Anti-reflection layer on the front (SiN)
• • Screen printing front side grid / firing
• Plating of nickel / formation of Ni _x Si _y thermal / plating of copper
• • Cover tin for soldering

The 6A - 6F show steps to make a solar cell 1''' according to a fourth embodiment.

In contrast to the preceding embodiments, the gallium here forms both a back field 4 as well as a permanent passivation layer 10 out.

The gallium 8th is here analogous to the second embodiment over the entire surface directly on the silicon substrate 2 at its substrate back 3 applied. In contrast, however, the gallium is here in a common step to the formation of the gallium back field 4 driven and to form a Passivierschicht 10 oxidized. With the passivation layer 10 Here is the backs passivation of the solar cell 1''' educated.

• • Sägeschadenätze
• • Alkalische Textur
• • Emitterdiffusion
• • Kantenisolation/Phosphorglasätze/Rückseitenpolitur
• • Aufbringen des Galliums vollflächig auf der Rückseite (PVD, CVD, Sputtern, Tauchen, Tintenstrahldruck, Siebdruck)
• • Thermisches Eintreiben und Oxidieren des Ga zur gleichzeitigen Ausbildung eines Ga-BSFs mit Dicke > 500 nm und einer Ga2O3-Schicht
• • Laserablation
• • Selektiver Emitter
• • Antireflexschicht auf der Vorderseite (SiN)
• • Siebdruck Vorderseiten-Grid/Feuern
• • Platen von Nickel/Bildung von Ni_xSi_y thermisch/Platen von Kupfer
• • Deckschicht Zinn zum Löten

Thus, according to this embodiment, the manufacturing method may comprise the following steps by way of example:
Example of a PERC process (single-crystal wafer) in which Ga forms both a BSF and a passivation layer

• • Saw damage sets
• • Alkaline texture
• • emitter diffusion
• • Edge insulation / Phosphor glass etchings / Backside polish
• Full application of gallium on the reverse side (PVD, CVD, sputtering, dipping, inkjet printing, screen printing)
• • Thermal driving and oxidation of the Ga for the simultaneous formation of a Ga-BSF with thickness> 500 nm and a Ga2O3 layer
• • laser ablation
• • Selective emitter
• • Anti-reflection layer on the front (SiN)
• • Screen printing front side grid / firing
• Plating of nickel / formation of Ni _x Si _y thermal / plating of copper
• • Cover tin for soldering

In all four embodiments, the application of the gallium onto the wafer can also be effected by means of spin coating. For this purpose, a certain amount of Ga melt is applied to the rotating, over 30 ° C hot wafer. Due to the centrifugal force, the melt is distributed evenly over the wafer. Of course, gallium can also be applied as an emulsion or solution of one of its compounds.

For application and driving there are also preferred alternatives: For driving gallium from a Ga2O3 / GaCl3 sacrificial passivation, the application by CVD and driving by flash lamp is an effective combination. For pure gallium, spincoating and firing are effective.

As silkscreen pastes for metallization especially silver-containing pastes come into question. These are fired at temperatures of 800-900 ° C and do not form a eutectic with silicon.

Although the present invention has been fully described above with reference to preferred embodiments, it is not limited thereto but is modifiable in a variety of ways.

For example, the formation of the gallium back field is also possible for bifacial solar cells. For this, the full-surface backside metallization is replaced by a finger layout.

The metallization can be done both locally (LCO's) and over the entire surface in different embodiments. In further embodiments, simultaneous metallization of both sides of the silicon substrate 2 be provided, which represents a particularly economical alternative. This variant is particularly suitable for the production of bifacial solar cells.

For example, it is also necessary for a solar cell, which uses a substrate with p-type doping and in which the emitter is formed on the back, on the front side of a so-called p-type front surface field. For this purpose, the gallium-doped area can also be completely or locally at the front, d. H. be designed as front panel or front surface field.

Further, a gallium highly doped region may also be formed as an emitter on the front side. If it is an n-type wafer (i.e., the base is n-type doped), then the emitter on the front side is a p + doped region. Here, gallium can be used as a doping element.

In general, the gallium or a gallium compound should not be exposed to a high-temperature step during production. Only gallium doped silicon, that is, a gallium backside field, can be exposed to high temperature steps.

LIST OF REFERENCE NUMBERS

1

solar cell

1'

solar cell

1''

solar cell

1'''

solar cell

2

Silicon substrate

3

Substrate back

4

Gallium back surface field

5

depth

6

Passivation layer

7

top layer

8th

gallium

8th'

arrears

9

recesses

10

Passivation layer

11

gallium

11 '

arrears

12

Passivation layer

13

nickel layer

14

copper layer

15

topcoat

d

thickness

e

eutectic

S

solidus

T

temperature

TSE

temperature

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 202015101360 U1 [0003]
• DE 202015103803 U1 [0005]

Claims (14)

Solar cell ( 1 ; 1'; 1''; 1''' ), with a silicon substrate ( 2 ), which has a substrate backside ( 3 ), wherein at the substrate rear side ( 3 ) a gallium back field ( 4 ) is provided. Solar cell according to claim 1, characterized in that the gallium back field ( 4 ) a gallium doping to a depth ( 5 ) of greater than 500 nanometers, in particular greater than 2 micrometers, in the silicon substrate ( 2 ) having. Solar cell according to one of the preceding claims, characterized in that a passivation layer ( 6 ; 10 ) at the back of the substrate ( 3 ) is provided. Solar cell according to claim 3, characterized in that the passivation layer ( 10 ) is formed as a gallium oxide layer. Solar cell according to one of the preceding claims, characterized in that the silicon substrate ( 2 ) has a gallium concentration greater than 10 ¹⁷ atoms per cubic centimeter, in particular greater than 10 ¹⁸ atoms per cubic centimeter. Solar cell according to one of the preceding claims, characterized in that the gallium back field ( 4 ) over the entire surface in the uppermost layer at the back of the substrate ( 3 ) of the silicon substrate ( 2 ) is provided. Solar cell according to one of the preceding claims, characterized in that the gallium back field ( 4 ) only in sections locally in the uppermost layer ( 7 ) at the back of the substrate ( 3 ) of the silicon substrate ( 2 ) is provided. Process for producing a solar cell ( 1 ), in particular a solar cell ( 1 ) according to claim 7, comprising the steps of: applying gallium ( 8th ) on one with recesses ( 9 ) structured passivation layer ( 6 ), which on the back of a silicon substrate ( 2 ) is provided; and driving the gallium ( 8th ) as doping in the silicon substrate ( 2 ) in the area of the recesses ( 9 ) for forming a local gallium backside field ( 4 ). Process for producing a solar cell ( 1'; 1''' ), in particular a solar cell ( 1'; 1''' ) according to any one of claims 1 to 6, comprising the steps of: applying gallium ( 8th ) over the entire surface directly on the back of the substrate ( 3 ) of a silicon substrate ( 2 ); and driving the gallium ( 8th ) as doping in the silicon substrate ( 2 ) for forming a gallium backside field ( 4 ). A method according to claim 8 or 9, characterized in that after the driving in a step of removing residues ( 8th' ) of the gallium and / or gallium-silicon alloy formed during driving. Method according to claim 9, characterized in that the gallium ( 8th ) during driving to form a passivation layer ( 10 ) is oxidized, in particular to a full-surface gallium (III) oxide layer. Process for producing a solar cell ( 1 ''), in particular a solar cell ( 1'' ) according to any one of claims 1 to 6, comprising the steps of: applying a gallium compound ( 11 ) passivation layer ( 12 ) directly onto the back of the substrate ( 3 ) of a silicon substrate ( 2 ); Driving gallium out of the passivation layer as doping into the silicon substrate ( 2 ) for forming a gallium backside field ( 4 ); and removing residues ( 11 ' ) of the gallium compound. Method according to one of claims 8 to 12, characterized in that the driving of the gallium ( 8th ) into the silicon substrate ( 2 ) is treated by high temperature, laser beam, flash lamp and / or firing. Method according to one of claims 8 to 13, characterized in that the gallium ( 8th ) when driving into a depth ( 5 ) greater than 500 nanometers, in particular greater than 2 micrometers, into the silicon substrate ( 2 ) is driven.

Images